Non-linear hydraulic shock absorbers are used in railroad cars to minimize damage from bumping and collisions and the units are referred to as cushion devices. During a 14 mile per hour collision between rolling stock weighing 220,000 pounds shock forces transferred to cushion devices can result in a rise in hydraulic fluid pressure to a peak of more than 1,200,000 pounds per square inch (psi) within just a few seconds. The very high peak pressures, combined with a wide range of operational shock absorbing create special engineering challenges. While it is necessary to rapidly release hydraulic fluid pressures in a cushion unit during high speed collisions to prevent an explosive failure, at lower speeds pressures build more slowly and if they are released too rapidly the railcar or its cargo can be damaged.
Typically rail car cushion devices mounted in rail cars (e.g., as disclosed in U.S. Pat. No. 5,325,700), are filled with hydraulic fluid. The cushion units are designed to absorb a constant force by using a piston to force hydraulic fluid through apertures spaced along the side walls of a central cylinder and into an annular space surrounding the circumference of the cylinder, i.e., between the casing and the cylinder. As the piston traverses the cylinder it progressively closes the apertures leaving fewer and fewer open. By varying the spacing along the cylinder wall an attempt has been made to create a relatively constant resistive force throughout the piston stroke. In railroad operation, forces applied to railcars during collisions are not necessarily linear or constant with respect to time. The piston is commonly returned to its original position by a spring or by charging the hydraulic fluid in the cushion device with nitrogen gas to a pressure of about 600 to 800 pounds psi. Typical cushion devices in the art suffer from the disadvantage that rapid shock force loading, even at a low total force, can result in damage to a railcar and cargo if hydraulic fluid escapes too rapidly through the apertures in the sidewalls of the cushion device cylinder.
Rail car collisions commonly occur at a variety of different speeds and involving different loads that can be more than 220-350 thousand pounds. Common rail car gas charged cushion devices are fabricated with either a 10-inch or 8-inch cylinder and piston, and with a 10-inch stroke. (An 8-inch diameter 15-inch stroke unit is currently under consideration for approval by the American Association of Railroads.) The two common 8- and 10-inch diameter units must currently accommodate all the different types of shock absorbing requirements encountered in normal railroad operations. Theoretically, pressure in a hydraulic cushion device is related to at least the area of the piston; the working area of the cushion device; the volume of fluid in the cylinder; the pressure of gas in the cylinder; the temperature of the fluid (e.g., heat generated from rapid fluid movement); the frictional forces in the device; the hydraulic fluid viscosity and the change in viscosity with temperature; the collision rate (i.e., velocity/time); the rail car mass; and, the variation in collision applied force(s) with time (i.e., a multi-equation, multivariate analysis). The analysis is further complicated, because in common cushion devices, as a shock absorbing piston traverses a cylinder it pushes hydraulic fluid through apertures and progressively closes the apertures that are available for release of pressure. Thus, while certain "educated" guesses have been made as to the number, position and size of apertures in hydraulic cylinder devices, usage has shown deficiencies resulting in both cargo damage, metal fatigue, and failure of cushion devices.
The inventor has disclosed test devices for railcar cushion devices, and parameters determining hydraulic damping coefficients in railcar cushion devices in U.S. Pat. No. 5,325,700 (incorporated herein by reference).
Service life of cushion units is varied, depending upon type of use, number of high impact collisions and the like. Moving parts are subject to wear, (e.g. seals), while the casing and metal parts are subject to corrosion and metal fatigue. Generally cushion units give several years of service before failing. Conventional cushion units are, unfortunately, a compromise that may absorb shock forces well at some collision velocities and within certain load limits, but may commonly fail at other velocities or loads. If satisfactory for higher speed higher mass collisions the units may be too "stiff" for lower speed or lower load collisions; and if satisfactory for lower speed lower load collisions too "relaxed" for higher speed/load collisions. Limitations in cushion devices restrict the types of railroad operations that may be conducted with railcars containing e.g., fragile cargo.
It is an object of the invention to provide cushion unit that are less of a compromise, and are capable of absorbing shock more uniformly over a wider range of operating conditions, loads and collision velocities.